Sunlight Hours Analysis - Long Calculation Time, sort of...

Hi, 

I have an observation, followed by a question/ 

I've been using the Sunlight Hours Analysis component on a large model. The model context is nearly 16,000 surfaces, and there are c. 350 surfaces for the sunlight hours analysis. 

I'm producing BRE Annual Probable Sunlight Hours calculations and so to match the BRE approach, I'm using 100 sun vectors, each representing 1% of probable sunlight hours. I could use the Sunpath and Analysis Period components to produce sun positions for the whole year, but this gives results that do not fully reflect the BRE methodology - which is important here. I'm detailing this just to clarify that this isn't a full annual calc of 8760 hours for 350 surfaces. 

Anyway, when I run the calc, it takes about an hour to run, but the Sunlight Hours Component itself reports a calculation time of 3 seconds! Does this mean that the rest of the time is all about prepping the brep geometry? If so, is there a reason why this is much slower than when using a view of sky recipe and exporting to radiance. For the same project, I completed a view of sky calculations and based on the number of test points and -ad setting, this was completing about 5.25 billions rays so I understand why that took an hour. 

Any thoughts as to why the sunlight hours calc seems to take so long? 

thanks

Nick